Apparatus for coating fibers with binder to produce fiberboard

Abstract

An arbor having arms is rotated in a drum such that the ends of the arms sweep the inner wall of the drum and advance a mass of fibers or chips as a flowable fleece along the drum from an inlet to an outlet thereon. The inside wall of the drum is provided with at least one V-shaped baffle extending radially in from this wall and directed in the direction of displacement of the fleece so that it forms a dead-fluid region and a vortex street in the fleece. The binder is injected into the fleece at the dead-fluid region and mixes with the fibers in the vortex street. The baffle has relative to the arbor rotation axis a tangential length and a radial height both many times greater than the average fiber length.

Description

FIELD OF THE INVENTION

The present invention relates to an apparatus for coating fibers or chips. More particularly this invention concerns a system for producing a mass of binder-coated fibers or chips for the production of fiberboard.

BACKGROUND OF THE INVENTION

The fibers, here including chips or other elongated pieces of material used in the production of construction chipboard or fiberboard (pressed board), are usually fed into a horizontal cylindrical drum at one end. This drum is rotated or an axially extending arbor in the center of the drum is rotated so that arms on this shaft displace the fibers axially toward an outlet. Binder in liquid form is injected into the drum usually through nozzles which open either radially or tangentially relative to the inner surface of the drum.

In order to prevent the fibers from being damaged, the rotation speed is generally maintained high enough so that the mass of fibers forms a cylindrically hollow body or fleece lying against the inside of the drum, with the ends of the advancing or mixing arms serving to mix the binder with the fibers. Too slow a rotation speed causes the fibers to tumble and break. The so coated fibers are then formed into mats as discussed in the commonly assigned U.S. Pat. No. 3,655,098 of Ulrich Schnitzler. The mats are then pressed into rigid boards.

As a rule it is almost impossible to achieve a uniform binder coating over each fiber. Even though it is possible in such devices to mix the binder well with the fibers, many fibers are frequently left wholly or partially uncoated, which causes weaknesses in the finished board. Excessive mixing does little to remedy this situation, and often destroys the fibers.

OBJECTS OF THE INVENTION

It is therefore an object of the present invention to provide an improved apparatus for the coating of fibers.

Another object is the provision of an improved system which uniformly coats fibers for the production of fiberboard.

A further object is to provide a coating system which overcomes the above-given disadvantages.

SUMMARY OF THE INVENTION

These objects are attained according to the present invention in a system wherein the interior of the coating drum is provided with at least one baffle which forms in the flowable fleece in the drum a dead-fluid region. A nozzle in this dead-fluid region injects binder into the flowable fleece being displaced from the inlet to the outlet of the drum by the arms of an arbor which sweep the inner wall of the drum. In this manner a vortex street is formed in back of the baffle, which according to this invention has a tangential length and a radial height which are both substantially greater than the average fiber length.

In accordance with further features of this invention the binder is injected at a rate (volume/unit time) which is between 45 and 55 percent the rate necessary to eliminate separation in back of the baffle. This principle is discussed at pages 9-28 and 9-29 of the Handbook of Fluid Dynamics edited by V. Streeter (McGraw Hill: 1961). In this manner a vortex street is maintained downstream in back of the baffle for best mixing of the binder and the fibers.

DESCRIPTION OF THE DRAWING

The above and other objects, features, and advantages of the invention will become more readily apparent from the following description, reference being made to the accompanying drawing in which:

FIG. 1 is a longitudinal section partially in diagrammatic form through the apparatus according to this invention;

FIG. 2 is a cross section taken along line II--II of FIG. 1;

FIG. 3 is a view of the detail indicated by arrow III of FIG. 2;

FIG. 4 is a view in the direction of arrow IV of FIG. 3;

FIG. 5 is a sectional view similar to FIG. 4 indicating another form of this invention; and

FIG. 6 is a perspective view illustrating the functioning of the present invention.

SPECIFIC DESCRIPTION

As shown in FIGS. 1 and 2 the apparatus comprises basically a cylindrical treatment drum 1 formed with an inlet 2 in which fiber from a hopper 21 is fed and formed with an outlet 3 whence the coated fiber leaves. An arbor 4 extending along the axis A of the drum 1 has a plurality of axially staggered and diametrically opposed arms 5 whose ends sweep the interior of the drum as the arbor 4 is rotated about the axis A by a drive 6. This relative rotation, which could also be effected by rotating the drum concurrently with or instead of the arbor 4, forms all of the individual fibers F (FIG. 3) into a flowable fleece 9 lining the inside of the drum 1. Two injectors 7 axially between the arms 5 at the upstream end of the drum 1 are connected through a pipe 18 and a pump 19 to a source 20 of fluid binder.

FIGS. 3 and 4 show how each injector 7 is formed of a generally V-shaped baffle 10 having an apex directed against the direction D of displacement of the arms t and of the fleece 9 and having front surfaces 10a extending radially to the axis A. Located between the flanks of this baffle 10 is a pair of nozzles 8 directed back in the direction D. The baffle has a length T measured tangentially to the drum 1 and a height R measured radially which are both multiples of the average length L of the fibers F. The relationship ##EQU1## is maintained. Here R .apprxeq. 6L, and T .apprxeq. 5L.

FIG. 6 shows the fleece 9 as if it were planar. Here it can be seen that a dead-fluid region 11 is formed in back of the baffle 10. A pair of vortices 16 are formed from the normally laminar flow 15 and are shed periodically to form a Karman vortex street 17 as discussed on page 9-6 of the above-cited Handbook of Fluid Dynamics.

An injector 13 as shown in FIG. 5 can be used in place of the injector 7 of FIG. 4. This arrangement is a single piece of metal of triangular section having a pair of sides 10' constituting the baffle and a bore 12 terminating in a pair of nozzle orifices 8' opening in a back face 14 of the injector 13 in flow direction D. The sides 10' have like but opposite inclinations, here 25.degree., to the direction D.

EXAMPLE

Fibers having a length between 1 mm and 5 mm, a width between 0.5 mm and 2 mm, and a thickness between 0.1 mm and 0.3 mm are used in fine fiberboard, the average length (L) being 3 mm. In coarser fiberboard the chips have a length between 3 mm and 15 mm, a width between 2 mm and 8 mm, and a thickness between 0.4 mm and 0.5 mm, the average length being 10 mm. Thus the dimensions T and R of the baffle 10 and 13 lie between 30 mm and 35 mm. The flow speed in direction D is between 20 m/sec and 35 m/sec.

Claims

We claim:

1. An apparatus for coating fibers with a binder, said apparatus comprising:

a drum having an axis and an axially spaced inlet and outlet;

an arbor extending axially in said drum and having axially spaced radially extending arms with ends adjacent the inner wall of said drum;

at least one baffle on said inner wall between axially spaced arms having a height in a radial direction and a length in a tangential direction, each many times greater than the average length of said fibers, said baffle having a pair of sides inclined to the direction of displacement of said fleece thereby and terminating in a common apex pointing opposite this displacement direction;

means for relatively rotating said arbor and said drum with said ends of said arms sweeping said inner wall for displacing said fibers from said inlet to said outlet as a fluid fleece lying in a layer of limited thickness along said wall and forming a dead-fluid region in back of said baffle;

at least one nozzle on said inner wall of said drum in said dead fluid region directly in back of said baffle and between said sides; and

means for introducing binder into said drum through said nozzle, said fleece being advanced at a rate to form a vortex street in back of said dead-fluid region, said binder being injected at a volume rate per unit time equal to generally half the rate sufficient to eliminate said vortex street.

2. The apparatus defined in claim 1 wherein the radial height divided by the fiber length is greater than three and less than seven.

3. The apparatus defined in claim 1 wherein the tangential length of said baffle divided by the fiber length is greater than three and less than seven.

4. The apparatus defined in claim 1 wherein said drum is non-rotatable and said arbor and arms are rotatable about said axis.